The use of radiating coaxial cables in these environments is particularly important as a result of the development of mobile communication systems (radio links, cellular phone, cordless telephone, wireless computer network, etc.).
Nowadays, these mobile communications systems operate in a very large spectrum the frequencies of which are allocated at an international level. Starting from the low frequencies, the bands are allocated as follows (these figures are only indicative and may vary according to countries):                74 to 87 MHz: Private mobile radio;        88 to 108 MHz: FM radio broadcast;        145 to 175 MHz: Private mobile radio;        around 225 MHz: Digital Audio Broadcast (DAB);        380 to 470 MHz: Private mobile radio and TETRA networks;        824 to 894 MHz: TDMA IS-54 and CDMA IS 95 mobile networks;        870 to 960 MHz: GSM 900, GSM R and TETRA mobile communication networks;        1710 to 1880 MHz: GSM 1800 networks;        1885 to 2200 MHz: UMTS networks.        
Moreover, such radiating coaxial cables can also be used in outdoor or indoor environments to restrict the radio coverage in a narrow lateral corridor along an axis, e.g. a transport route, a railway, a defined path in a workshop, etc. Restricting the radio coverage in a certain width may be required to avoid interference with neighbouring transmitters operating at the same radio frequency.
Various types of radiating cables are known; they consist of a coaxial cable comprising an inner conductor surrounded by a dielectric and an outer conductor of tubular form. The outer conductor includes apertures which generate an electromagnetic radiation. The outer conductor is covered by an insulating outer sheath.
The apertures in the outer conductor may be of various types, for example a longitudinal slot over the entire length of the cable, or numerous small holes very close to each other. There also exist cables in which the outer conductor consists of a loose braiding, or sometimes of a layer of wires wound in a spiral around the dielectric. The common characteristic of these cables is that the total length of the outer conductor includes apertures separated by a distance considerably shorter than the wavelength of the radiated signal. All these cables operate in a mode known as “coupled mode” and the radiated energy propagates in a direction parallel to the cable. With these cables, the signal received by a receiving antenna falls off rapidly when the distance between the antenna and the cable increases. Moreover, the received signal fluctuates greatly when the receiving antenna is moved along a path parallel to the cable.
A more recent technique has proposed cables known as “radiated mode cables” in which the outer conductor includes groups of apertures, which are reproduced with a constant spacing s, this spacing being of the same order of magnitude as the wavelength of the signal to be radiated. The radiation produced by the radiated mode cables propagates in a radial direction (FIG. 1), forming an angle θ1 with the cable axis lying between 0° and 180°.
It is known by those skilled in the art that a radiated mode cable produces a main mode which propagates in a direction forming an angle θ1 with the axis of the cable; this angle is given by the formula:θ1=arcos{λ/s−√εr}  (1)                where:        s: aperture group spacing (in meters);        λ: signal wavelength in the air (in meters);        εr: relative dielectric constant of the cable (coefficient).        
It may be noted here that for practical purposes the expression wavelength “in the air” and wavelength “in free space” can be considered as synonyms.
In the above expression, the direction of reference for measuring θ1 is the direction of the cable end fed by the radio frequency generator, as illustrated in FIG. 1.
A radiated mode cable operates in this way in a band from λstart to λend where λstart and λend correspond to θ1=0° and 180° respectively. These wavelengths (in the air) λstart and λend are linked respectively to the frequencies fstart and fend (in MHz) by
                              f          start                =                  300                      λ            start                                              (        2        )                                          f          end                =                  300                      λ            end                                              (        3        )            
It is known by those skilled in the art that the ratio fend/fstart is given by
                                          f            end                    ⁢                      /                    ⁢                      f            start                          =                                            √                              ɛ                r                                      +            1                                              √                              ɛ                r                                      -            1                                              (        4        )            
With the dielectric usually used between the inner and outer conductors, √εr is generally lying between ≅1.1 and ≅1.15. Consequently, fend/fstart varies between ≅14 and ≅21.
Hereinafter most calculations are carried out with √εr=1.136 which is the most frequent value with dielectrics presently used. It should be stressed, however, that the conclusions which will be drawn will generally also be valid if √εr is not equal to this particular value.
FIG. 1 shows the graph of θ1 versus f/fstart calculated for √εr=1.136. This figure shows that θ1 begins at 0° when f is equal to fstart. Then, θ1 increases with f up to 180° (degrees from 0 to 180 are shown in FIG. 1 on the inner part of the curve) when f=fend which is equal to 15.71 fstart (1 to 15.71 are shown in increments in FIG. 1 on the outer part of the curve). Below fstart and above fend, the cable operates in coupled mode.
Compared to coupled mode cables, the main advantages of the radiated mode cables are:                a lower coupling loss;        a coupling loss which increases less rapidly in the radial direction;        a field which fluctuates less when moving parallel to the axis of the cable.        
However, it is also known by those skilled in the art that the third advantage above disappears when the frequency reaches 2fstart if some precautions are not adopted since there appears a second order mode which propagates in a direction θ2 different from θ1 and which interferes with the main mode. According to the relation (1), θ1≅94° (for √εr=1.136) when f=2fstart. If f continues to increase, a third mode appears when f=3fstart and so on for all the fstart multiples. As a consequence, the higher the frequency, the more numerous are the secondary modes which all propagate in different directions θi. These interferences between the main and secondary modes result in rather large field strength fluctuations along the cable.
If we consider first the case of narrow band radiating cables, i.e., the cables used at only one or several frequencies very close to each other (this is the case if the cable is only used for one radio communication application listed above), prior art cables were generally designed to have the θ1 angle very close to 90° in the frequency band for which the cable is intended. The main reasons are avoiding the secondary mode which appears for θ1 higher than about 94° and also because, with most aperture types, the radiation decreases in the directions nearly parallel to the cable axis (i.e., with θ1 close to 0° or 180°).
Formula (1) indicates that choosing a spacing s≅λ gives rise to θ1≅90° as √εr≅1. This is the reason why prior art narrow band radiating cables are designed with the aperture group spacing approximately equal to the wavelength (in the air) for which the cable is intended.
FIG. 2 illustrates a specific exemplary embodiment of such prior art narrow band radiating cables; in this exemplary embodiment, each aperture group includes two slots slanted in opposite directions and the group spacing is approximately equal to the wavelength λ.
If we consider now the case of wide band radiating cables, i.e., cables which must feature satisfactory performances in the frequency band allocated to several mobile communication applications, the main problem to solve is the field fluctuations due to the interference produced by the secondary modes described earlier. Several solutions have been proposed to cancel or to reduce to an acceptable level the intensity of the secondary modes, at least on a frequency band from 2fstart to k×fstart where k depends on the efficiency of the solution. Generally, k varies from 3 to 5 or even 7 with the best solutions. If we refer to FIG. 1, this means that the performances deteriorate (there are large field strength fluctuations along the cable) if θ1 exceeds 115°, 135° or 145°, with k equal to 3, 5 and 7 respectively, and with √εr=1.136.
It must be mentioned that if √εr≅1.1, the θ1 values which correspond to k equal to 3, 5 and 7 are respectively 114°, 133° and 143°; these values are close to those obtained for √εr=1.136. Similar conclusions apply if√εr≅1.2.
FIG. 1 also shows that θ1 raises very rapidly from 0° to 35° when f increases from fstart to 1.1fstart. This band is too narrow to be of any interest in practice and it results that prior art wide band radiating cables are generally designed to have θ1 lying between ≅35° and an angle θmax ranging between 115° and 145° (θmax depends on the efficiency of the solution used to cancel or attenuate the secondary modes) in the frequency bands for which they are intended. This also means that (in the best case) the direction θ1 into which the wave generated by the radiating cable propagates, lies within an angle of about 110° centred on the direction perpendicular to the cable axis.
As a consequence, prior art wide band radiating cables are designed by choosing the aperture group spacing s in order to have θ1 lying between ≅35° and θmax in the frequency bands for which the cable is intended. Such cables can be used at frequencies where θ1>θmax, but the performances deteriorate due to the interferences between the main mode and insufficiently attenuated secondary modes.
The following specific documents illustrate the state of the art referred to here above.
German Patent No. 2 812 512 describes a pattern which, with the aim of producing a periodic profile in the direction of the radiating cable axis consists of apertures of the same size and of the same shape, the density of which varies periodically along the cable. As the holder of this patent indicates, the purpose of such a pattern is to produce a periodic profile of the radiation intensity in the direction of the axis of the cable. Moreover, this document does not give the extent of the frequency band in which the secondary modes are attenuated.
United Kingdom Patent Application No. 1 481 485 describes a periodic pattern consisting of two main slots and four auxiliary slots. The auxiliary slots are arranged on either side of each of the main slots. In this device, the secondary modes appearing at the frequencies lying between fstart and 5fstart are negligible or almost zero. Moreover, a pattern of greater size would include ten slots and, consequently, would be difficult to produce in practice, since the total length of the apertures would be such that it would weaken the mechanical strength of the outer conductor.
French Patent Application No. 2 685 549 describes a pattern including N apertures, the useful frequency band of which lies between fstart and N×fstart.
The patterns described in these last two documents have the drawback that the apertures are present over almost the whole length of the cable, which has the effect of reducing the mechanical strength. It is well known, in fact, that deformations of the cable or of the apertures in the outer conductor may greatly affect the performances obtained. Another drawback of these known solutions is the difficulty of producing long slanted slots with different inclinations on certain types of cable constructions.
German Patent No. 9 318 420 describes a solution which uses a corrugated outer conductor. No mention is made of the elimination of secondary modes.
European Patent No. 0765002 describes a solution for a narrow band cable which uses a periodic pattern consisting of two opposed slots elongated in the axial direction. The pattern spacing is approximately equal to one wave length in order to radiate in a direction θ1 close to 90°.
U.S. Pat. No. 6,292,071 describes a solution for a wide band coupled mode cable which uses groups of apertures separated by a spacing varying between 8 and 10 m. Such an exemplary embodiment has the drawbacks of the coupled mode cables.
International Patent Publication No. 99/17401 describes a solution for a radiated mode cable which is based on a principle similar to the one shown in FIG. 2 but in which each slanted slot is replaced by a group of circular or elongated holes.
Belgian Patent No. 1010528 describes a radiating cable operating in radial direction for a specific frequency band, which comprises an outer conductor provided with a periodic pattern of aperture groups with a spacing p equal toλ/(√ε+1),                where λ is the wavelength of the lowest frequency at which the cable operates in radiated mode and εr is the dielectric constant of the cable. The length of the periodic pattern is equal to p/2 and the number of apertures in each group ranges from 1 up to 10.        